1. Field of the Invention
The present invention relates to improvements in winged aircraft and more particularly to an aircraft in which the relative disposition of wing to fuselage can be changed (yawed) in flight to optimize the aircraft's capability of flying at both transonic and supersonic speeds.
2. Discussion of the Prior Art
It is well known that the airframe configuration requirements for efficient supersonic flight are not compatible with the airframe configuration requirements for efficient slow speed flight, take-off and climb, or descent and landing. For low speed flight, and conventional take-off and landing, the optimum wing planform is generally considered to be a long span, narrow chord wing having little, if any, sweep angle.
Since the total lift developed by a lifting airfoil, with other factors such as angle of attack and dynamic pressures being equal, is substantially dependent on the aspect ratio of the airfoil, defined as the square of the span of the airfoil divided by the surface area thereof, it is apparent that a long narrow wing is capable of developing substantially greater lift-to-drag ratio than is attainable using a short broad wing of the same plan area. The use of the high aspect ratio wing offers the advantages that the angle of attack required for landing and take-off is at the low end of the spectrum. The take-off and landing speeds are lower than for low aspect ratio wings, thus permitting a relatively short take-off and landing, as well as a low speed climb to altitude. Furthermore, the drag due to lift is also at the low end of the spectrum, thereby providing high aerodynamic efficiency for subsonic cruise and low power requirement during take-off and landing.
For transonic and supersonic flight however, highly swept wings are considered preferrable because aerodynamic drag may be greatly reduced thereby, and other advantages are also obtained. For example, even during high altitude subsonic cruise the highly swept wing configuration develops a comparatively low drag coefficient, while still developing the required lift coefficient. It has been experimentally shown that lift/drag ratios of 10 to 12 may be obtained with the highly swept wing at supersonic high altitude cruise thus making such flights economically feasible even in the case of commercial transport aircraft. The highly swept wing configuration is also preferred for supersonic flight at low levels, where the combination of high dynamic pressure at the high frequency end of the gust spectrum may establish the structural strength requirements of the aircraft, since the gust loads imposed on a highly swept wing are much smaller than on a more or less straight wing due to a smaller change in lift force resulting from change in the angle of attack. This result is due to the fact that a moving aircraft experiences atmospheric turbulence only as the result of sudden changes in the angle of attack which may be said to be in the direction of the resultant of the vertical component of gust velocity and horizontal component of aircraft velocity.
However, a swept wing aircraft designed solely on the basis of supersonic high performance flight will obviously not perform satisfactorily for subsonic cruise, take-off and landing. Even present day supersonic aircraft are designed with aspect ratios higher than that considered optimum for supersonic cruising flight in order to make take-off and landing feasible. These supersonic aircraft must also climb to cruise altitude at subsonic speeds to prevent heavy shock wave ground damage and they must do this at the expense of increased fuel consumption since the relatively low aspect ratio of the wing results in increased drag due to lift while in the climb. For example, it is not unusual for a supersonic swept wing transport on a transatlantic flight to expend 30% or more of its total fuel requirement during take-off and climb to cruise altitude at subsonic speed.
Various attempts have been made to enable the wing configuration of an aircraft to be modified in flight so as to optimize both the low speed and high speed performance of the aircraft. Examples of these so-called variable geometry aircraft, in which the sweep back is modified by moving the wings relative to the fuselage in simple or compound motions, are disclosed in the U.S. Patents to Alfred, Jr. et al., U.S. Pat. No. 3,053,484; Halliwell, U.S. Pat. No. 3,133,716; Jacquart et al., U.S. Pat. No. 3,381,918; Willox, U.S. Pat. No. 3,405,280; Jacquart et al., U.S. Pat. No. 3,405,891; and Whitener et al. U.S. Pat. No. 3,447,761. Such examples include devices which swivel each wing about pivots so as to effect a transition from sharp sweep back suited for high speeds, to smaller sweep back for obtaining the necessary lift at low speeds.
These solutions, however, have the inherent disadvantages that the swiveling of the wings results in a shift in the center of pressure of the aerodynamic forces exerted thereon as well as in a displacement of the center of gravity of the aircraft. Furthermore, the position of the center of lift is effected by the flight speed with the transition from subsonic to supersonic speed notably resulting in a large rearward shift of the center of pressure of the force exerted on the wing. In addition, the structural components necessary to accommodate a wing pivoted at a point near one of its ends requires the use of massive bearings which must carry the wing root bending moment.
The use of an airframe configuration having a single fuselage with a main wing movable to selected positions depending upon flight characteristics is also known. Vogt U.S. Pat. No. 3,155,344 provides an aircraft having a single fuselage with two sets of rotatably mounted integral wings, one set for supersonic flight having a small area, and another set for subsonic flight having a relatively large area. Hubschman U.S. Pat. No. 1,740,016 discloses an aircraft wherein the wing-to-fuselage angle of separate left and right wings can be independently adjusted by the pilot using a hand crank, with FIG. 6 thereof showing a skewed relationship between the wings and the fuselage. Crook U.S. Pat. No. 3,258,228 discloses an aircraft wherein the payload unit can be trimmed to a different attitude from that of a flight unit, the latter including a wing configuration. The flight unit can be held offset relative to the payload unit, for example, to maintain aerodynamic efficiency in case of a cross wind.
The idea of turning the wing as a whole with respect to a single fuselage by providing a wing pivotally attached to the fuselage so that it can be set at right angles to the fuselage for take-off, landing, and low speed flight, and pivoted as a unit so that is is skewed with one side of the wing swept forward and the other swept back at high speeds has also been proposed. See applicant's papers entitled "Theoretical Determination of the Minimum Drag of Airfoils at Supersonic Speeds," Journal of the Aeronautical sciences, Vol 19, No. 12, December 1952, pp. 813-822; and "Aerodynamic Design for Supersonic Speeds", Proceedings of the First International Congress in the Aeronautical Sciences, Madrid, September 1958, Advances in Aeronautical Sciences, Pergamon Press, N.Y., 1959, pp. 34-51.
Studies of the stability and control of oblique or skewed winged aircraft have also been made. See the paper by Campbell, J.P. and Drake, H.M., "Investigation of Stability and Control Characteristics of an Airplane Model with Skewed Wing in the Langley Free-Flight Tunnel," TN 1208, 1947, NACA. The advantages of an "all wing" aircraft arranged so that it can be steered to fly at varying oblique angles have been considered. Note Lee, G.H., comments appended to Kuchemann, D., "Aircraft Shapes and Their Aerodynamics," Proceedings of the 2nd I.C.A.S. Advances in Aeronautical Sciences, Vol. 3-4, Pergamon Press, N.Y., 1962, pp. 221-252; and Lee, G.H., "Slewed Wing Supersonics," The Airplane, Vol. 100, Mar. 3, 1961, pp. 240-241.
Applicant's previously mentioned patent, U.S. Pat. No. 3,737,121, discloses an airframe in which a parallelogram principle is utilized to achieve an efficient selective angular disposition between a pair of airfoils (a main wing and a horizontal stabilizer) and a pair of fuselages. The main wing and the horizontal stabilizer form one set of parallel sides of the parallelogram while the two fuselages form the other two sides. The two airfoils are pivoted with respect to the spaced fuselages and enable two important in-flight changes in aircraft configuration to be effected: (1) the skewing or yawning of the airfoils relative to the direction of flight for high speed flight; and (2) the lateral spreading of the weight distribution to minimize the bending stresses of the wing. The increased extension of the aircraft components in the fore and aft direction serves further to reduce the drag at supersonic speed, and the upwardly curved main wing configuration compensates for any roll tendency caused by the yawed positioning of the wing.